UNIVERSITY  OF  CALIFORNIA  PUBLICATIONS 

COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


THE  COMMON  HONEY  BEE  AS 

AN  AGENT   IN    PRUNE 

POLLINATION 


BY 

A.  H.  HENDRICKSON 

Instructor  in  Pomology 


BULLETIN  No.  274 

December,  1916 


UNIVERSITY   OF  CALIFORNIA   PRESS 

BERKELEY 

1916 


Benjamin  Ide  Wheeler,  President  of  the  University. 

EXPERIMENT    STATION    STAFF 

HEADS  of  divisions 

Thomas  Forsyth  Hunt,  Director. 

Edward  J.  Wickson,  Horticulture  (Emeritus). 

Herbert  J.  Webber,  Director  Citrus  Experiment  Station;  Plant  Breeding. 

Hubert  E.  Van  Norman,  Vice-Director;   Dairy  Management. 

William  A.  Setchell,  Botany. 

Myer  E.  Jaffa,  Nutrition. 

Robert  H.  Loughridge,  Soil  Chemistry  and  Physics   (Emeritus). 

Charles  W.  Woodworth,  Entomology. 

Ralph  E.  Smith,  Plant  Pathology. 

J.  Eliot  Coit,  Citriculture. 

John  W.  Gilmore,  Agronomy. 

Charles  F.  Shaw,  Soil  Technology. 

John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 

Frederic  T.  Bioletti,  Viticulture  and  Enology. 

Warren  T.  Clarke,  Agricultural  Extension. 

John  S.  Burd,  Agricultural  Chemistry. 

Charles  B.  Lipman,  Soil  Chemistry  and  Bacteriology. 

Clarence   M.   Haring,  Veterinary   Science   and   Bacteriology. 

Ernest  B.  Babcock,  Genetics. 

Gordon  H.  True,  Animal  Husbandry. 

James  T.  Barrett,  Plant  Pathology. 

Fritz  W.  Woll,  Animal  Nutrition. 

A.  V.  Stubenrauch,  Pomology. 

Walter  Mulford,  Forestry. 

W.  P.  Kelley,  Agricultural  Chemistry. 

H.  J.  Quayle,  Entomology. 

D.  T.  Mason,  Forestry. 

J.  B.  Davidson,  Agricultural  Engineering. 

Elwood  Mead,  Rural  Institutions. 

H.  S.  Reed,  Plant  Physiology. 

William  G.  Hummel,  Agricultural  Education. 

John  E.  Dougherty,  Poultry  Husbandry. 

S.  S.  Rogers,  Olericulture. 

Frank  Adams,  Irrigation  Practice. 

H.  S.  Baird,  Dairy  Industry. 

David  N.  Morgan,  Assistant  to  the  Director. 

Mrs.  D.  L.  Bunnell,  Librarian. 


Division  of  Pomology 
A.  V.  Stubenrauch  A.  H.  Hendrickson 

W.  L.  Howard  W.  P.  Tufts 

R.  H.  Taylor  Richard  Schmidt 

W.  L.  Sweet 


THE  COMMON  HONEY  BEE  AS  AN  AGENT  IN 
PRUNE  POLLINATION* 


BY 

A.  H.  HENDRICKSON 


During  the  past  three  years  the  Division  of  Pomology  of  the 
University  of  California,  and  the  Horticultural  Commissioner  of  Santa 
Clara  County  have  been  conducting  a  number  of  experiments  in  an 
attempt  to  solve  some  of  the  problems  connected  with  the  pollination 
of  plums  and  prunes.  A  part  of  this  work  was  done  at  the  University 
Farm  at  Davis  and  a  part  in  the  Santa  Clara  Valley.  These  experi- 
ments were  undertaken  to  endeavor  to  find  out  why  some  plums  bore 
abundant  crops  under  certain  conditions,  while  the  same  varieties 
under  different  conditions  were  either  sterile  or  produced  light  crops. 
The  question  of  what  effect  the  interplanting  of  certain  varieties  had 
upon  the  size  of  the  resulting  crops  also  seemed  to  be  of  utmost  im- 
portance, especially  in  view  of  the  repeated  inquiries  concerning  this 
factor  which  have  been  received  at  this  office. 

As  a  result  of  these  investigations,  the  Division  of  Pomology  has 
accumulated  a  mass  of  data  which  may  help  to  elucidate  some  of 
the  problems  now  confronting  the  plum  and  prune  growers  of  this 
state.  Among  the  most  interesting  results  obtained  during  the  season 
of  1916  were  those  relating  to  the  common  honey  bee  as  a  pollen- 
carrying  agent.  The  experiment  demonstrated  to  all  who  saw  it  that 
the  honey  bee  is  one  of  the  most  important  agencies  in  transferring 
pollen  from  tree  to  tree. 

In  the  Santa  Clara  Valley,  where  this  part  of  the  experiment  was 
carried  on,  there  are  a  large  number  of  orchards  of  mixed  varieties. 
Wherever  the  French  prune  is  planted  next  to  either  the  Sugar  or 
Imperial  prunes,  there  seemed  to  be  a  difference  of  opinion  as  to 
whether  or  not  the  size  of  the  crop  was  in  any  way  affected  by  the 

*  The  following  bulletin  is  a  report  on  part  of  the  pollination  project  which 
has  been  carried  on  by  the  University  of  California  for  the  past  three  years. 
It  is  presented  with  the  idea  of  bringing  before  the  prune  growers  of  the 
state  the  importance  of  the  common  honey  bee  as  a  factor,  not  to  be  over- 
looked, in  the  problems  involved  in  prune  pollination.  Other  reports  will  be 
presented  from  time  to  time  as  data  warrant. 


128  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 

interplanting.  Some  growers  maintain  that  larger  crops  of  French 
prunes  are  produced  on  trees  planted  in  close  proximity  to  the 
Imperial  or  Sugar  varieties,  and  vice  versa.  Others  state  that  no 
difference  was  noticeable,  and  no  benefits  could  be  seen  as  a  result 
of  interplanting  varieties  for  purposes  of  cross-pollination.  It  was 
noticed  that  at  the  height  of  the  blossoming  period  there  seemed  to  be 
a  scarcity  of  insects,  especially  bees,  which  are  generally  credited  with 
being  responsible  for  the  transfer  of  pollen  from  one  flower  to  another. 
In  some  cases  the  writer  worked  for  days  among  the  blossoms  in  several 
large  prune  orchards  without  seeing  or  hearing  a  bee.  The  scarcity  of 
other  insects  which  might  have  worked  in  the  flowers  was  also  very 
noticeable.  In  most  orchard  sections  bees  can  be  found  working  upon 
fruit  trees  during  the  blossoming  season,  as  it  is  at  this  time  that  the 
supply  of  nectar  is  most  plentiful.  The  opposite  seemed  to  be  true 
in  certain  sections  of  the  Santa  Clara  Valley.  Although  the  weather 
was  favorable  for  bees  to  work  among  the  blossoms,  being  warm  and 
with  but  little  wind,  some  of  the  orchards  in  the  vicinity  of  Meridian 
seemed  to  attract  practically  no  bees  at  all. 

The  Santa  Clara  Valley  is  largely  planted  to  prunes,  principally 
the  French  or  Agen  variety,  with  smaller  interplantings  here  and  there 
of  the  Sugar  and  Imperial  varieties.  These  three  varieties  blossom  at 
nearly  the  same  time  and  should  theoretically  pollenize  one  another 
effectively.  Because  of  the  conflicting  reports  of  the  value  of  inter- 
planting varieties  and  the  noticeable  lack  of  bees,  it  was  decided  to 
find  out  in  an  experimental  way  what  effect  a  large  supply  of  bees 
would  have  upon  the  setting  of  fruit  on  some  typical  French  and 
Imperial  prune  trees.  The  experiment  was  planned  in  such  a  way 
as  to  have  the  bees  as  nearly  under  the  control  of  the  experimenter  as 
possible. 

Two  pairs  of  adjoining  French  and  Imperial  prune  trees,  as  nearly 
as  possible  of  the  same  age  and  size,  were  chosen  for  the  experiment. 
Over  each  pair  was  constructed  a  tent  of  white  mosquito  bar.  The 
tents  were  approximately  40  feet  long,  25  feet  wide,  and  16  feet 
high,  and  constructed  as  nearly  insect-proof  as  possible.  Aside  from 
the  fact  that  the  force  of  the  wind  was  broken  by  this  arrangement, 
the  trees  under  the  tents  were  under  approximately  the  same  conditions 
as  the  remainder  of  the  trees  in  the  orchard.  Both  tents  were  com- 
pleted before  any  of  the  blossoms  opened.  They  were  taken  down 
after  the  blossoms  had  reached  a  stage  where  there  was  no  further 
danger  of  being  pollinated  from  outside  sources.  As  soon  as  25  per 
cent  of  the  blossoms  had  opened,  a  hive  of  bees  was  placed  under  one 
tent  and  kept  there  throughout  the  blossoming  period  (about  five  days 


PRUNE    POLLINATION    STUDIES  129 

in  the  season  of  1916).  All  insects  were  excluded  from  the  second 
tent,  and  i  careful  watch  maintained  to  see  that  none  entered.  The 
bees  under  tent  number  1  started  to  work  almost  immediately  and  did 
not  seem  to  object  to  being  imprisoned  in  the  tent  or  try  to  escape. 
They  seemed  to  prefer  the  flowers  of  the  French  prune  to  those  of 
the  Imperial,  and  seemed  to  devote  most  of  their  attention  to  the 
former  variety.  The  hive  was  kept  under  the  tent  until  it  appeared 
that  all  of  the  blossoms  had  opened  and  it  could  reasonably  be  sup- 
posed that  nearly  every  blossom  had  been  visited  by  one  or  more  of 
the  bees.  The  bees  were  then  removed.  The  tents  were  taken  down 
after  the  petals  of  the  blossoms  had  fallen  and  there  was  no  danger 
of  pollen  being  brought  from  outside  sources  to  the  trees  in  the  experi- 
ment. As  practically  all  of  the  trees  in  the  orchard  blossomed  approx- 
imately at  the  same  time  as  the  trees  under  the  tents,  it  is  reasonable 
to  suppose  that  no  pollen  reached  the  trees  under  experiment,  after  the 
tents  had  been  removed. 

Results  were  apparent  as  soon  as  it  was  possible  to  tell  how  much 
fruit  had  set.  It  was  evident  that  the  French  prune  tree  under  the 
tent  with  the  bees  had  set  a  much  heavier  crop  than  that  of  the  best 
trees  exposed  to  normal  conditions.  The  French  prune  tree  under  the 
tent  from  which  bees  and  all  other  insects  were  excluded,  had  set  a 
very  light  crop  (fig.  1).  For  some  unaccountable  reason,  the  Imperial 
tree  under  the  tent  with  the  bees  set  only  a  light  crop  of  fruit.1  The 
Imperial  tree  under  the  tent  from  which  all  insects  were  excluded  set 
but  a  few  scattering  fruits. 

All  trees  were  carefully  watched  throughout  the  season.  At  an 
early  date  it  became  necessary  to  prop  the  French  prune  tree  which 
had  been  under  the  tent  with  the  bees,  to  prevent  serious  damage  to 
the  tree  by  breaking  the  limbs  (fig.  2).  The  difference  in  the  amount 
of  fruit  set  by  the  French  prune  which  had  been  under  the  tent  with 
the  bees  and  the  amount  borne  on  the  adjoining  trees,  all  in  the  same 
state  of  thrift  and  having  been  given  the  same  care,  was  easily  appar- 
ent to  even  the  casual  observer. 

Approximately  one  thousand  blossoms  were  counted  and  tagged 
on  each  tree.  The  record  of  the  fruit  resulting  from  these  blossoms  was 
kept,  and  from  these  results  the  percentage  of  fruit  to  set  and  mature 
was  calculated.  The  following  table  gives  the  percentages  of  fruit  that 
actually  matured : 


i  This  part  of  the  experiment  will  be  repeated  in  order  to  determine,  if 
possible,  why  the  Imperial  prune  did  not  respond  to  cross-pollination  in  a  similar 
way  to  the  French  prune. 


130 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


No.  of  blossoms 
in   count. 
April  11,  '16 

French  prune  tree  under  tent  with  bees 1069 

French  prune  tree  under  tent  from  which  bees 

were  excluded  1058 

French  prunes,  average  orchard  set  9991 

Imperial  prune  tree  under  tent  with  bees  1060 

Imperial  prune  tree  under  tent  from  which  bees 

were  excluded  1050 

Imperial  prune  tree,  average  orchard  set  2180 


No.  of  fruits 

that  matured. 

Aug.  1,  '16 

Per 
cent 

193 

18.05 

11 

1.04 

359 

3.59 

18 

1.69 

0 

0 

157 

7.20 

Fig.  1. — French  prune  tree  which  had  been  enclosed  in  a  tent  of  mosquito 


bar  from   which  insects  were  excluded   during  the  blossoming  period, 
graph  taken  August  1,  1916.     Compare  with  Fig.  4. 


Photo- 


A  glance  at  the  table  shows  plainly  that  the  amount  of  fruit  set 
per  one  hundred  blossoms  on  the  French  prune  was  much  greater  on 
the  tree  under  the  tent  with  the  bees  than  on  the  trees  under  normal 
conditions.  Only  a  very  small  percentage  of  fruit  matured  on  the 
tree  which  had  been  covered  by  the  tent  from  which  all  insects  were 
excluded.  In  the  case  of  the  Imperial  prunes,  the  tree  under  the  tent 
with  the  bees  set  only  a  light  percentage  of  fruit  for  reasons  which 
are  not  as  yet  understood.  The  tree  which  was  enclosed  in  the  tent 
from  which  all  insects  were  excluded  set  no  fruit. 


PRUNE    POLLINATION    STUDIES 


131 


The  results  would  seem  to  warrant  the  following  conclusions : 

1.  That  the  French  prune  may  be  aided  in  setting  a  satisfactory 

crop  by  the  presence  of  a  large  number  of  bees  in  the  orchard 
during  the  blossoming  period. 

2.  Without  the  aid  of  bees  or  other  insects,  the  set  of  fruit  on  the 

French  prune  is  often  light. 

3.  The  Imperial  prune  does  not  seem  to  be  able  to  set  fruit  unless 

pollinated  by  insects  with  pollen  from  other  trees. 


Fig.  2. — French  prune  tree  which  had  been  enclosed  in  a  tent  of  mosquito  bar 
in  which  a  hive  of  bees  was  kept  during  the  blossoming  period.  Photograph 
taken  August  1,  1916. 


It  should  be  stated  that,  although  the  crop  of  prunes  on  the  tree 
which  was  under  the  tent  with  the  bees  was  much  heavier  than  that  on 
the  tree  under  the  tent  from  which  bees  were  excluded,  the  size  of  the 
prunes  was  smaller.  The  prunes  were  also  smaller  than  the  average 
on  the  trees  not  in  the  experiment.  This  smaller  size  was  due,  no 
doubt,  to  the  exceptionally  heavy  crop  that  the  tree  was  compelled 
to  mature.  The  introduction  of  a  number  of  hives  of  bees  into  an 
orchard  would  not  necessarily  mean  that  as  heavy  a  crop  of  prunes 
would  be  produced  as  resulted  with  the  tree  in  the  experiment  under 


132  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION 

the  tent.  It  is  only  reasonable  to  suppose  that  the  bees  flying  some 
distance  from  the  hives  would  have  less  effect  on  the  setting  of  fruit 
in  the  orchard  as  a  whole,  than  did  those  whose  efforts  were  confined 
to  but  two  trees. 

Since  the  results  of  the  experiment  have  become  known,  consider- 
able discussion  has  arisen  as  to  the  best  method  of  testing  out  the 
work  of  bees  in  the  various  sections.  Some  growers  are  preparing  to 
purchase  a  number  of  swarms  of  bees  to  have  them  permanently  in 
their  orchards;  others  expect  to  try  to  keep  the  bees  in  the  foothill 
sections  for  most  of  the  year,  bringing  them  into  the  orchards  during 
the  blossoming  period  only ;  while  others  expect  to  try  the  plan  adopted 
in  parts  of  southern  California,  where  bee-men  having  large  numbers 
of  swarms  are  hired  to  place  their  bees  for  a  certain  length  of  time 
in  the  orange  groves.  It  seems  to  be  the  consensus  of  opinion  that  the 
best  work  would  result  if  the  bees  were  brought  in  from  some  outside 
district  and  scattered  about  the  orchards,  about  one  hive  to  the  acre, 
during  the  blossoming  period,  and  then  removed.  Some  of  the  growers 
are  at  present  giving  the  latter  plan  serious  consideration. 

The  results  of  this  experiment,  while  interesting,  do  not  necessarily 
solve  all  of  the  problems  involved  in  pollination,  but  serve  to  throw 
some  light  on  this  one  side  of  the  question.  It  is  the  purpose  of  this 
division  to  repeat  this  experiment  in  whole  or  in  part  and  in  con- 
nection with  other  experiments  now  in  progress,  the  station  may  be 
able  to  give  further  light  on  the  question  from  time  to  time. 


